It is known that the various petroleum hydrocarbons can be marked using colorants. However, there exists a need for invisibly marking petroleum-derived products in order to identify the various grades of fuels, to distinguish manufacturer's brands, and to make misuse impossible or at least traceable. In this regard, it is desirable that the added marker be readily detected by non-scientific personnel. Finally, the marker should be detectable at low enough levels so that the physical and chemical properties of the petroleum product are not appreciably altered. Historically, various problems have accompanied the use of dyes or colorants as markers for petroleum products, including sludging, crystallization, or agglomeration of the dye upon standing or storage.
U.S. Pat. Nos. 2,028,637; 2,925,333; 3,004,821; 3,164,449; 3,350,384; 3,435,054; 3,690,809; 3,704,106; 4,009,008; 4,049,393; 4,303,407; and 4,735,631; European Application No. 95 975; and U.S.S.R. Patent No. 297,659 describe the use of colorants and dyes in marking petroleum products.
Ger. Offen. 1,913,912; and U.S. Pat. Nos. 4,278,444, 4,992,204; and 5,279,967 describe visible or ultraviolet fluorescing compounds useful as markers in petroleum products. The marking or tagging systems based on UV fluorescence have the inherent disadvantage that many of the petroleum hydrocarbons themselves contain condensed aromatic compounds which fluoresce when exposed to UV radiation.
U.S. Pat. No. 5,201,921 describes a method for marking plastic with UV fluorescent compounds.
U.S. Pat. No. 4,540,595 teaches the marking of documents such as bank checks with certain fluorescent phenoxazine dyes.
U.S. Pat. No. 5,093,147 describes the use of polymethine infrared fluorescent compounds in bar codes.
U.S. Pat. No. 3,630,941 describes 16,17-dialkoxy-violanthrones vat dyes for use as infrared fluorescers for marking articles.
All of the above infrared fluorophores lack adequate solubility in most petroleum hydrocarbons to be suitable for such use.
This invention provides a method for marking or tagging various petroleum products, for identification purposes. Preferably, the markers of the present invention are squaraines, phthalocyanines, or naphthalocyanines which fluoresce in the near infrared region when exposed to near infrared light. Also provided are certain near infrared fluorophoric compounds which are soluble in petroleum hydrocarbons.